The present invention relates to centrifuges and, particularly to decanter type centrifuges. The invention is directed to a pump within a feed chamber of the centrifuge conveyor that redirects mist generated during operation of the centrifuge from about a feed pipe and back into the feed chamber.
A decanter centrifuge generally comprises an imperforate bowl mounted for rotation about its central longitudinal axis. The bowl typically includes a cylindrical section and a frusto-conical section at one end. A screw conveyor is coaxially mounted within the bowl and adapted for rotation at a differential speed with respect to the bowl. The screw conveyor typically comprises a coaxial central hub having a series of conveyor flights extending radially therefrom and forming a helix along the length of the hub.
The rotation of the bowl of the decanter centrifuge creates a centrifugal force which separates a liquid feed mixture or slurry into its constituent parts. The feed mixture within the bowl forms a cylindrical pond, with a ring or layer of the heavy constituent material(s) adjacent the inside bowl wall and a ring or layer of the lighter constituent material(s) radially inward of the heavy material layer.
A decanter centrifuge also includes a feed chamber which is typically incorporated within the conveyor hub. FIG. 1 illustrates a conventional feed chamber 2. The feed chamber 2 receives feed material from a feed pipe 4. The feed pipe 4 extends into the conveyor hub 6 and terminates within the feed chamber 2. As the feed material exits the feed pipe 4 it follows a path indicated by the arrows and engages an accelerator wall 8. As the feed material travels radially along the accelerator wall 8 it gradually accelerates. Thereafter, the feed material is transferred from inside the conveyor hub to the bowl 10 via passages 11. The accelerator wall 8 is generally smooth and allows the feed material to be brought up to speed relatively gently. In this configuration, the feed chamber 2 includes an entry wall 12 opposed to the accelerator wall 8. The entry wall 12 is defined by a frusto conical sidewall 14 and an end wall 16. The end wall 16 extends radially inward from the sidewall 14 to define an opening or bore 18 to the feed chamber 2.
In some instances, the volume rate of feed material to be accelerated becomes too great for the feed chamber and the feed material does not contact a sufficient amount of the accelerator wall 8 to bring the feed material up to speed quickly enough. In these instances, the feed material level increases radially inward causing feed material to move axially along the sidewall 14. Mist generated by the acceleration of the feed material or the feed material itself may overflow the feed chamber 2 into an adjacent portion 20 of the conveyor hub through the space between the feed pipe 4 and the end wall 16 of the feed chamber. If enough feed material accumulates in the conveyor hub to cause an unbalance in the conveyor, the decanter will shut down due to excessive vibration. Also, if the rotating feed material builds up in the space surrounding the feed pipe and contacts the stationary feed pipe 4, the feed pipe could potentially break. The configuration of FIG. 1 is shown as including vanes 22 extending from the sidewall 12. These vanes 22 are used to assist in accelerating the feed material up to speed and preventing overflow through the space surrounding the feed pipe 4 and into the conveyor hub. However, mist may still be created and move axially in the feed chamber and through this space, settling in a portion of the conveyor hub adjacent to the feed chamber.
One form of decanter centrifuge is shown in U.S. Pat. No. 3,885,734 to Lee. The centrifuge includes a conveyor that includes a central hub extending for a portion of the longitudinal length of the bowl. A feed pipe extends along the axis of the conveyor terminating in a feed chamber within the hub. A feed slurry is introduced into the feed chamber via the feed pipe. The slurry engages an accelerator vane for imparting radial and tangential velocity to the slurry. Once the slurry is brought up to speed, it is discharged out of the feed chamber through a feed passage and into the centrifuge bowl. The centrifuge also includes a partition within the hub about the feed pipe and an annular seal (not shown) that closes the space between the partition and the feed pipe.
U.S. Pat. No. 5,551,943 to Leung et al. discloses a decanter centrifuge including a feed pipe, positioned at one end of the conveyor hub. The feed chamber is separated from an adjacent portion of the conveyor hub by a feed pipe baffle. The feed pipe baffle is positioned toward one end of the conveyor hub and is provided to prevent the feed slurry from flowing from the feed chamber, back along the inside surface of the conveyor hub and into the adjacent portion of the conveyor hub.
U.S. Pat. No. 3,428,246 to Finkelston discloses a centrifuge including a feed chamber having a baffle plate that extends axially from an interior circumferential surface of the conveyor hub towards, but not abutting, a feed tube. A first annular member extends radially outward from the feed tube. A second annular member extends radially inward from the baffle plate. Each of the radially extending members includes a radially extending flange. The radially extending flanges are positioned in an overlapping relationship so as to provide a deflector assembly which is intended to guard against the passage of feed toward a chamber on the opposite side of the baffle.
U.S. Pat. No. 5,354,255 to Shapiro discloses a decanter centrifuge including a feed tube mounted within a conveyor hub that terminates within an open feed chamber. The feed chamber is separated from an adjacent hub section by a baffle plate. A plurality of vanes extend from the baffle plate into the feed chamber. The vanes redirect feed material traveling axially along the feed tube away from the accelerator portion.
U.S. Pat. No. 3,405,866 to Amero discloses a centrifuge including a main feed compartment and a secondary feed compartment inside the conveyor hub. A baffle acts as a splash guard separating the main feed compartment from the secondary feed compartment. The feed pipe passes through an opening within the baffle.
U.S. Pat. No. 4,816,152 to Kalleberg discloses a separator including an inlet pipe terminating in a receiving compartment. The receiving compartment includes impeller plates positioned about the inlet pipe. The rotating impeller plates force excess feed liquid back into the receiving compartment and out of holes positioned in the upper wall of the receiving compartment.
The invention relates to centrifuges and is preferably directed to a mist pump for a decanter centrifuge conveyor adapted for rotation about its longitudinal axis. The conveyor includes a feed chamber housing the mist pump. The feed chamber comprises a sidewall and a lip extending radially inward from a distal end of the sidewall. The sidewall includes an interior surface and is coaxially positioned about the longitudinal axis of the conveyor. The lip terminates at a circumferential surface, the circumferential surface defining an opening to the feed chamber. The feed chamber also includes a ring wall defined by an inner diameter defining an opening in the ring wall and an outer diameter. The ring wall is normal to the longitudinal axis, parallel to the lip and spaced radially inward from the sidewall. The feed chamber further includes a plurality of vanes positioned between and connected to the ring wall and the lip, the vanes being spaced radially inward from the sidewall.
The mist pump of the present invention may be incorporated into an existing feed chamber for redirecting mist, generated during acceleration of the feed material that gravitates towards a portion of the conveyor hub adjacent to the feed chamber.
In addition to the centrifuges that utilize an enclosed feed chamber, there are also conveyors, for example a conveyor described in U.S. Pat. No. 5,354,255 (incorporated herein by reference), that use an open feed chamber to reduce sudden feed acceleration. The open feed chamber allows the liquid to reach the pond without contacting feed passages or vanes between the feed chamber and the bowl. While these types of centrifuges allow the feed material to more gradually and smoothly achieve a desired speed than closed chamber configurations, mist may still be generated in the feed chamber. The present invention is intended to operate with both closed and open feed chamber centrifuges.